1. Field of the Invention
This invention relates generally to the field of electrical signal processing and more specifically to the field of automatic frequency alteration and control.
2. Description of the Prior Art
Devices for detection and correction of frequency variation are commonly known. Although such devices have almost unlimited utility, they are particularly useful in tape recorder applications which require a constant frequency relationship between original recording and playback.
In previously known systems, time base fluctuations in recorded signals have been alleviated by simultaneously recording a reference tone and the signal, using different channels of the tape recorder. When the precise frequency of the reference tone is known, the rate at which the tape in the recorder is moving is easily determined. On playback, the taped reference frequency becomes an input to the playback recorder to insure that the tape speed is proper to produce true frequency reproduction of the original recorded signal. A representative example of the above is U.S. Pat. No. 4,123,773 to Rotter et al. which utilizes a feedback loop to control minor fluctuations in tape drive speed.
While the above process appears to be easily executed, difficulties often arise in the physical implementation. Many recorders without the built-in servo capability necessary to compare reference tone frequency with tape speed require cumbersome follow-on processing equipment. Often, even tape recorders with servo/reference tone capabilities operate only at specific tone frequencies different from the frequency recorded on the tape which is being used. These recorders also require the full time-base compensation burden to be placed on reference tone circuitry in follow-on processing equipment. Conventionally implemented tracking reference circuitry typically uses a bank of band-pass filters, each centered on a different frequency. Filter selection is done via front panel control. The output of the selected filter is then fed into a phase locked loop (PLL) designed to lock on that particular frequency tone. Difficulty arises because the PLL must operate at a different center frequency if either the tape recorder playback speed or the tone frequency is changed. This requires either duplicating circuitry (multiple PLLs) or changing of PLL parameters. Both of these solutions are awkward, require operator assistance, and presume that the playback operator is aware of which reference tone frequency is recorded on any particular tape. This latter presumption is often incorrect, for it is common for a recording operator to use a reference tone which is some multiple or sub-multiple of the expected frequency. Devices disclosed in U.S. Pat. No. 3,546,354 to Uchiyama include apparatus which automatically synchronize input pitches between the tape and the original source. A major disadvantage of these apparatus is that they must be readjusted each time that the reference tone frequency is altered.
In an attempt to standardize reference tones for industrial applications, the Inter-Range Instrumentation Group (IRIG) has published suggested tape speed/reference tone relationships as follows:
______________________________________ Tape Speed (ips) Reference Tone (kHz) ______________________________________ 15.0 25.0 7.5 12.5 3.75 6.25 1.875 3.125 ______________________________________